Make double parsing locale independent (and faster)

[lkeegan]

• use std::from_chars instead of strtod to parse doubles if available (c++17 required)
  • otherwise falls back to previous strtod implementation
  • std::from_chars is locale independent and faster than strtod
• add more double parsing tests
• add double parsing google benchmarks
• resolves parsing of doubles depends on locale #1566

[rikardn]

std::from_chars has the potential of being the fastest possible parsing function. If I remember correctly the implementation of this in libstdc++ was very delayed or even never implemented. I remember using a third party implementation once. Has it been implemented now in both gcc and clang?

[lkeegan]

• std::from_chars is significantly faster than strtod: https://quick-bench.com/q/sBL71XcFMu2N8MGvh5s_GoNHxD8
• this gives ~20% reduction in overall parse time in the benchmarks of expressions with many doubles

[lkeegan]

std::from_chars has the potential of being the fastest possible parsing function. If I remember correctly the implementation of this in libstdc++ was very delayed or even never implemented. I remember using a third party implementation once. Has it been implemented now in both gcc and clang?

According to https://github.com/fastfloat/fast_float their implementation is now part of gcc (12) and llvm (not sure exactly what version)

[rikardn]

Ok, nice. That was the implementation I was using. I think clang 15 supports it.

[certik]

Awesome, thanks!

[lkeegan]

So it seems that just checking for c++17 at compile time was not sufficient as standard library support for from_chars (including parsing floating point values) is incomplete for:

• libstdc++ < 11
• macOS < 13.3
• libc++ < 14(?)

I've switched to just directly bundling and using the fast_float library instead (which is the underlying fast implementation of std::from_chars - thanks @rikardn!) and is available as a single header file.

[lkeegan]

@isuruf would you have time to take a look at this at some point? It would be nice to resolve #1566